Takes ages then fails with List remote refs failed: com.jcraft.jsch.JSchException: verify: false

This is the first time I have used Team City. It is connected to git. It hasn't yet successfully done a build.
The process starts with the log entry "building clean patch for caching etc"
Many hours later after looking like it is doing stuff e.g. lots of log entries such as

[2013-05-03 10:17:34,057]  DEBUG [ical executor 4] - .git.GitVcsSupport.Performance - [getRemoteRefs] repository: git@myserver:myproject.git#develop {instance id=1, parent id=1}, took 586ms
and

[2013-05-03 03:57:30,781]  DEBUG [rmal executor 9] - .vcs.git.patch.GitPatchBuilder - Add file file.name, size 1433 bytes

it eventually logs this:

[2013-05-02 21:13:18,071]  DEBUG [ical executor 2] -      jetbrains.buildServer.VCS - VCS connection problem in build configuration MyConfig :: MyConfig: Failed for the root 'myroot' #1: List remote refs failed: com.jcraft.jsch.JSchException: verify: false
jetbrains.buildServer.vcs.VcsException: List remote refs failed: com.jcraft.jsch.JSchException: verify: false
 at jetbrains.buildServer.buildTriggers.vcs.git.OperationContext.wrapException(OperationContext.java:134)
 at jetbrains.buildServer.buildTriggers.vcs.git.GitVcsSupport.getRemoteRefs(GitVcsSupport.java:497)
 at jetbrains.buildServer.buildTriggers.vcs.git.GitVcsSupport.getCurrentState(GitVcsSupport.java:128)
 at jetbrains.buildServer.buildTriggers.vcs.git.GitCollectChangesPolicy.getCurrentState(GitCollectChangesPolicy

It then seems to start doing stuff again, but after yet more hours there is another of the above error. In total it has been running for days doing this.
Help!